Sift confectioners' sugar and cornstarch into a small bowl. Line a 9 x 9 x 2-inch pan with foil. Coat with nonstick cooking spray. Sift 2 tbsp of the confectioners' sugar mixture into pan, tilting to coat sides. Leave any excess in pan.
Place 2/3 cup water in a large bowl and sprinkle with gelatin. Soften for 5 minutes.
Meanwhile, combine granulated sugar, corn syrup and salt in medium heavy-bottomed saucepan. Cook over medium heat until sugar is dissolved, stirring occasionally, about 7 minutes. Strain into bowl with gelatin.
Beat on high speed with an electric mixer until light and fluffy, about 12 minutes, adding flavoring and food coloring, if desired (cherry and red for pink ones, lime and green for green). Spread into prepared pan and smooth top. Dust with 2 tbsp of the confectioners' sugar mixture. Let stand 2 hours until set.
Lift marshmallow from pan with foil. With a wet knife, cut into 4 squares. Cut each square into 12 pieces. Spread remaining confectioners' sugar mixture onto sheet pan. Toss marshmallows in sugar mixture; let stand 1 hour. Store in an airtight container at room temperature.